This invention relates to a combustion chamber for internal combustion engines of the direct injection type, and more particularly to a combustion chamber which can reduce the level of noise (hereinafter referred to as the noise generated by combustion) and can also reduce the amount of nitrogen oxide (hereinafter referred to as NOx).
It is well known in the field of the art that in internal combustion engines if the timing of injection of fuel into the combustion chamber is delayed to some extent so as to reduce the explosion pressure and the rate of increase thereof the level of the noise generated by fuel combustion can be reduced and at the same time the combustion gas temperature can be lowered thereby reducing the amount of NOx.
However, there encounters a difficulty that in general when the timing of fuel injection is delayed the thermal efficiency of the Diesel cycle will be lowered so as to increase the fuel consumption rate, and so imperfect combustion is caused thereby producing black exhaust gas or smoke.
Stated in brief, the fuel consumption rate is a function of dM/dt.multidot.S.multidot.Tm.multidot.Td.multidot.d.theta./dt, and in the similar manner the color of exhaust gas, NOx, the noise generated by combustion are a function of dM/dt.multidot.S.multidot.Tm.multidot.Td.multidot.d.theta./dt, respectively.
A delay in injection timing serves to reduce an ignition lag Td, and therefore as a mean cycle gas temperature Tm and a heat generation coefficient d.theta./dt are reduced the velocity of mixing fuel and air dM/dt and the time and space distribution "S" of mixing ratio will be reduced.
Accordingly, if the timing of fuel injection is delayed the level of noise generated by fuel combustion and the amount of NOx can be reduced; however, the fuel consumption rate will be increased thereby generating black exhaust gas or smoke based on the above-mentioned relationship.
In order to eliminate such difficulty it is only necessary to increase the air and fuel mixing speed dM/dt at the time of delaying the fuel injection timing or increase the combustion speed after the fuel ignition. Stated more specifically, the emission of black exhaust gas and increase in fuel consumption rate are due to the fact that the combustion process is maintained even after the piston has reached its top dead centre, and so in order to eliminate the above-mentioned disadvantage, it is necessary to increase the combustion speed thereby shortening the combustion period. In the conventional combustion chamber having a toroidal cavity formed in a piston head, in order to achieve a good combustion of fuel injected from fuel injection nozzles, pertinent combination of fuel injection angles in lateral and vertical directions with speed of air swirl can be selected.
In order to increase the combustion speed, it is required to increase the air flow (in particular, the air flow after the fuel ignition) within the combustion chamber.
The term "air flow" employed herein is meant by a large swirl motion generated in the combustion chamber and a resultant turbulence of a small scale.
To produce the above-mentioned air flow, there is conventionally provided a quadrilateral combustion chamber having a quadrilateral cavity formed in the upper part or top of a piston.
The combustion chamber of this form enables air turbulences of a small scale to be produced by a large air swirl motion in the corner square parts thereof so that a strong air flow can be produced therein.
However, the combustion chamber of such shape is disadvantageous in that the intensity of the turbulence of a small scale generated in the corner square parts and that of the large swirl are contrary to each other. In brief, if radius "R" of the corner square parts is comparatively smaller than radius "Ro" of the inscribed circle of the quadrilateral cavity (if the ratio R/Ro is reduced) the intensity of air turbulences of a small scale generated in the corner square parts can be increased; therefore the intensity of the large air swirl motion will be reduced. If the ratio R/Ro is increased, the intensity of the large air swirl can be increased, whilst the air turbulence of a small scale generated in the corner square parts will be reduced.
In any case, the combustion chamber of the conventional shape cannot afford increased turbulences of a small scale and the large air swirl motion at the same time, and further the turbulences can be produced only in the corner square parts so that a sufficiently large air flow cannot be produced therein.